Enhanced rate PSA process

ABSTRACT

High product recovery and low BSF are achieved for fast-cycle shallow adsorbers in VPSA gas separation enabled by the coupled effects of high intrinsic adsorption rate and proper particle size selection.

This application claims priority to prior U.S. Provisional Application60/256,485 filed Dec. 12, 2000 which is incorporated by reference in itsentirety herein.

FIELD OF THE INVENTION

This invention relates to pressure swing adsorption (PSA) processesusing adsorbents having high intrinsic adsorption rates. Moreparticularly, the invention relates to PSA processes wherein highproduct recovery and low bed size factor (BSF) are achieved forfast-cycle shallow adsorbers.

BACKGROUND OF THE INVENTION

There has been significant development of the various PSA, VSA and VPSAmethods for air separation over the past thirty years, with majoradvances occurring during the last decade. Commercialization of theseprocesses and continued extension of the production range can beattributed primarily to improvements in the adsorbents and processcycles, with advances in adsorber design contributing to a lesserdegree. Highly exchanged lithium molecular sieve adsorbents, asillustrated by Chao in U.S. Pat. No. 4,859,217, are representative ofadvanced adsorbents for oxygen production. Advanced adsorbents of thetypes mentioned above are the result of improvements in equilibriumproperties.

Improving process efficiency and reducing the cost of the lightcomponent product can be accomplished by decreasing the amount ofadsorbent required and by increasing the product recovery. The former isgenerally expressed in terms of bed size factor (BSF) in lbsadsorbent/TPDO (ton per day of contained O₂), while the latter is simplythe fraction of light component (i.e. oxygen) in the feed (i.e. air)that is captured as product. Improvement in adsorbents and reduction incycle time are two primary methods of reducing BSF.

While shorter cycles lead to shorter beds and higher adsorbentutilization, product recovery generally suffers unless adsorption rateis increased. This phenomena can be ideally characterized in terms ofthe size of the mass transfer zone (MTZ), i.e. the mass transfer zonebecomes an increasing fraction of the adsorbent bed as the bed depthdecreases. Since the adsorbent utilization with respect to the heavycomponent (e.g. nitrogen) is much lower in the MTZ than in theequilibrium zone, working capacity declines as this fraction increases.

The effect of particle size upon the size of the MTZ is conceptuallystraightforward in a single long adsorption step where a contaminant inrelatively low concentration is removed from the feed stream on thebasis of its higher equilibrium affinity to the adsorbent. When theadsorbate/adsorbent combination is characterized by a favorableisotherm, a steady state transfer zone is envisioned that moves throughthe adsorber at a constant speed. Distinct equilibrium and mass transferzones can be identified in the process. Under such conditions, and whenthe resistance to mass transfer is dominated by intraparticle porediffusion, it has long been recognized that reducing the adsorbentparticle size results in higher adsorption rates and smaller masstransfer zones. Unfortunately, pressure drop across the adsorbent bedincreases with decreasing particle size and leads to difficulty inparticle retention in the bed and an increased tendency forfluidization.

This ideal concept becomes blurred when the isotherms are unfavorableand/or the mass transfer zone is continuously developing or spreadingthroughout the adsorption step. Adding the remaining minimum steps ofdepressurization, desorption and pressurization to create a completeadsorption process cycle further complicates the behavior and characterof the mass transfer zone. Nevertheless, the idealized concept of theMTZ has been applied in the prior art as a basis to affect improvementsin process performance.

Ackley et al. (WO 99/43416) have shown increased performance in PSA airseparation processes through increased adsorption rates and larger masstransfer coefficients. This was accomplished while employing adsorbentsof high effective pore diffusivity (D_(p)≧5×10⁻⁶ m²/s) in conjunctionwith short cycles and shallow beds. Ackley et al. (WO 99/43418) extendedthese concepts to low pressure ratio cycles.

Jain (U.S. Pat. No. 5,232,474) discloses improving adsorbent utilizationby decreasing the adsorbent volume and/or increasing the product purity,wherein the removal of H₂O and CO₂ prior to cryogenic air separation isdescribed using a pressure swing adsorption (PSA) process. The adsorberis configured entirely with alumina or with layers of alumina and13×molecular sieve adsorbents. Smaller particles (0.4 mm to 1.8 mm) areused to achieve a smaller bed volume.

Umekawa (JP Appl. No. 59004415) shows a lower pressure drop and smalleradsorber for air purification by using a deep layer of large particles(3.2 mm) followed by a shallow layer of small particles (1.6 mm) of thesame adsorbent. The bed size and pressure drop of this layeredconfiguration are lower than for beds constructed either of all 3.2 mmor all 1.6 mm particles. The 1.6 mm particles occupy only a smallfraction (low concentration part) of the mass transfer zone in thelayered configuration.

Miller (U.S. Pat. No. 4,964,888) has suggested using larger particles(>14 mesh or 1.41 mm) in the equilibrium zone and small particles (<14mesh) in the mass transfer zone. This reduces the size of the MTZ whileminimizing the excessive pressure drop increase that would occur if onlysmall particles were used in both zones. Cyclic adsorption process timesgreater than 30 s are indicated.

Garrett (UK Pat. Appl. GB 2 300 577) discloses an adsorption apparatuscontaining particles in the size range between 6 mesh (3.36 mm) and 12mesh (1.68 mm) deployed in either discrete layers or as a gradient ofsizes with the largest particles located near the feed inlet and thesmallest particles located downstream near the outlet of the adsorber inboth configurations.

Very small adsorbent particles (0.1 mm to 0.8 mm) are necessary for thefast cycles and high specific pressure drop that characterize a specialclass of processes known as rapid pressure swing adsorption (RPSA).Typical RPSA processes have very short feed steps (often less than 1.0s) operating at high feed velocities, include a flow suspension stepfollowing the feed step and generally have total cycle times less than20 s (often less than 10 s). The behavior of the adsorption step is farremoved from the idealized MTZ concept described above. In fact, theworking portion of the bed is primarily mass transfer zone with only arelatively small equilibrium zone (in equilibrium with the feedconditions) in RPSA. A major portion of the adsorber is in equilibriumwith the product and provides the function of product storage. The highpressure drop (on the order of 12 psi/ft)/short cycle combination isnecessary to establish an optimum permeability and internal purging ofthe bed which operates continuously to generate product.

RPSA air separation processes using 5A molecular sieve have beendescribed by Jones et al. (U.S. Pat. No. 4,194,892) for single beds andby Earls et al. (U.S. Pat. No. 4,194,891) for multiple beds. Jones hasalso suggested RPSA for C₂H₄/N₂, H₂/CH₄, H₂/CO and H₂/CO/CO₂/CH₄separations using a variety of adsorbents. The RPSA system is generallysimpler mechanically than conventional PSA systems, but conventional PSAprocesses typically have lower power, better bed utilization and higherproduct recovery.

In somewhat of a departure from the original RPSA processes, Sircar(U.S. Pat. No. 5,071,449) discloses a process associated with asegmented configuration of adsorbent layers contained in a singlecylindrical vessel. One or more pairs of adsorbent layers are arrangedsuch that the product ends of each layer in a given pair face eachother. The two separate layers of the pair operate out of phase witheach other in the cycle. The intent is for a portion of the product fromone layer to purge the opposing layer—the purge fraction controlled byeither a physical constriction placed between the layers and/or by thetotal pressure drop across a layer (ranging from 200 psig to 3 psig).Particles in the size range of 0.2 mm to 1.0 mm, total cycle times of 6s to 60 s, adsorbent layer depths of 6 inches to 48 inches and feed flowrates of one to 100 lbmoles/hr/ft² are broadly specified. An optionalbimodal particle size distribution is suggested to reduce interparticlevoid volume. The process is claimed to be applicable to air separation,drying, and H₂/CH₄, H₂/CO and H²CO/CO₂/CH₄ separations.

Alpay et al. (Chem. Eng. Sci., 1994) studied the effects of feedpressure, cycle time, feed step time/cycle time ratio and productdelivery rate in RPSA air separation for several ranges of particlesizes (0.15 mm to 0.71 mm) of 5A molecular sieve. His study showed thatprocess performance was limited when adsorbent particles were either toosmall or too large. This was because ineffective pressure swing, lowpermeability and high mass transfer resistance (due to axial dispersion)were limiting at the lower end of particle size range, while high masstransfer resistance became limiting due to the size of the particles atthe larger end of the particle size spectrum. Alpay found maximumseparation effectiveness (maximum O₂ purity and adsorbent productivity)for particles in the size range 0.2 mm to 0.4 mm.

RPSA is clearly a special and distinct class of adsorption processes.The most distinguishing features of RPSA compared to conventional PSAcan be described with respect to air separation for O₂ production. Thepressure drop per unit bed length is an order of magnitude or morelarger and the particle diameter of the adsorbent is usually less than0.5 mm in RPSA. Total cycle times are typically shorter and the processsteps are different in RPSA. Of these contrasting features, pressuredrop and particle size constitute the major differences.

Other patents suggest the use of small particles in conventional PSAprocesses. Armond et al. (UK Pat. Appl. GB 2 091 121) discloses asuperatmospheric PSA process for air separation in which short cycles(≦45 s) are combined with small particles (0.4 mm to 3.0 mm) to reducethe process power and the size of the adsorbent beds. Oxygen of 90%purity is produced under the preferred cycle times of 15 s to 30 s andparticle sizes of 0.5 mm to 1.2 mm.

Hirooka et al. (U.S. Pat. No. 5,122,164) describes 6, 8 and 10-step VPSAprocesses for separating air to produce O₂. While the main thrust ofthis patent is the cycle configuration and detailed operation of thevarious cycle steps to improve yield and productivity, Hirooka utilizessmall particles to achieve faster cycles. A broad particle range isspecified (8×35 US mesh or 0.5 mm to 2.38 mm), but 12×20 US mesh or 0.8mm to 1.7 mm is preferred. Half-cycle times of 25 s to 30 s areindicated (total cycle times of 50 s to 60 s).

Hay et al. (U.S. Pat. No. 5,176,721) also discloses smaller particles toproduce shorter cycles, preferably in air separation. A vertical vesselwith horizontal flow across the adsorbent is depicted. Broad rangecharacteristics include particles less than 1.7 mm diameter, cycle timesbetween 20 s-60 s and pressure drop across the adsorbent less than 200mb (2.85 psig).

An alternative configuration includes an upstream portion of theadsorbent bed with particles of size greater than 1.7 mm, in which casethe particle fraction smaller than 1.7 mm comprises 30% to 70% of thetotal adsorbent mass. The aspect ratio of the bed (largest frontallength to bed depth ratio) is specified to be between 1.5 and 3.0. Smallparticle fraction alternatives of 0.8 mm to 1.5 mm and 0.4 mm to 1.7 mmare also given, as well as adsorbent pressure drop as low as 50 mbar(0.7 psig).

Wankat (CRC Press, 1986; Ind. Eng. Chem. Res., 1987) describes a conceptthat he terms “intensification” whereby decreased particle diameter isemployed to produce shorter columns and faster cycles. Bynon-dimensionalizing the governing mass balance equations for theadsorption process, a set of scaling rules are suggested which preservethe performance of the process in terms of product recovery, purity andpressure drop while increasing the adsorbent productivity. Thesetheoretical results are based upon the similarity of dynamic adsorptionbehavior (at the same dimensionless times and column locations). Thesimilarity concept presumes an idealized constant pattern MTZ, with thelength of the mass transfer zone (L_(MTZ)) directly proportional to thesquare of the particle diameter when pore diffusion is controlling.Furthermore, decreasing L_(MTZ) increases the fraction of bed utilized.Wankat indicates that increasing (L/L_(MTZ)) beyond a value of two tothree, where L is the bed depth, results in minimal improvement in thefractional bed utilization. A layer of small-size particles placed ontop of a layer of large-size particles is also suggested as a way tosharpen the mass transfer front. Some of the practical limitations tosmaller scale and faster operation have been noted and includefluidization, column end effects, wall channeling and particle sizedistribution. The intensification concept was later extended to includenon-isothermal and non-linear equilibrium effects in PSA processes byRota and Wankat (AIChE J., 1990).

Moreau et al. (U.S. Pat. No. 5,672,195) has suggested higher porosity inzeolites to achieve improved O₂ yield and throughput in PSA airseparation. A preferred porosity range of 0.38 to 0.60 is claimed inconjunction with a minimum rate coefficient. Moreau states thatcommercially available zeolites are not suitable for their inventionsince porosity is lower than 0.36.

Lu et al. (Sep. Sci. Technol. 27, 1857-1874 (1992); Ind. Eng. Chem. Res.32: 2740-2751 (1993)) have investigated the effects of intraparticleforced convection upon pressurization and blowdown steps in PSAprocesses. Intraparticle forced convection augments macropore diffusionin large-pore adsorbents where the local pressure drop across theparticle is high and where the pores extend completely through theparticle. The higher intraparticle permeability is associated with highparticle porosity, e.g. porosities (ε_(p))=0.7 & 0.595.

OBJECTS OF THE INVENTION

It is therefore an object of the invention to increase efficiency,reduce cost and extend the production range of high performanceadsorption processes for the separation of gases. It is a further objectof the invention to increase efficiency, reduce cost and extend theproduction range of high performance adsorption processes for productionof oxygen.

SUMMARY OF THE INVENTION

The invention is based upon the unexpected finding that mass transferrates can be significantly increased by increasing the product of theparticle porosity (ε_(p)) and effective macropore diffusivity (D_(p)) inconjunction with a decrease in particle size. For the purposes of thisinvention, the product (ε_(p)D_(p)) is termed the “intrinsic rateparameter” or “intrinsic rate property.” The synergistic effect obtainedfrom increasing (ε_(p)D_(p)) simultaneously with a decrease in theparticle diameter (d_(p)) results in a much larger increase in the masstransfer rate coefficient than can be achieved by manipulating any one,or combination of two, of these parameters. This increase in masstransfer rate can be applied to affect a significant improvement inseparation performance. In a preferred embodiment an adsorption processuses an adsorbent zone comprising an adsorbent selected from the groupconsisting of A-zeolite, Y-zeolite, NaX, mixed cation X-zeolite, LiXhaving a SiO₂/Al₂O₃ ratio of less than 2.30, chabazite, mordenite,clinoptilolite, silica-alumina, alumina, silica, titanium silicates andmixtures thereof, wherein said adsorbent has a mass transfer coefficientfor nitrogen of k_(N2)≧12 s⁻¹ and an intrinsic rate property for N₂,when measured at 1.5 bar and 300K, of (ε_(p)D_(p))≧1.1×10⁻⁶ m²/s. Otherpreferred embodiments include the development of process parametersaround which such materials should be used and preferred methods forincreasing the intrinsic rate parameter.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages will occur to those skilled inthe art from the following description of preferred embodiments and theaccompanying drawings, in which:

FIG. 1 is a schematic diagram showing the apparatus used to measureintrinsic adsorption rate.

FIG. 2 is a graph showing the effect of the intrinsic rate parameter andmass transfer rate coefficient upon product recovery and bed size factorfor a 60 s cycle;

FIG. 3 is a graph showing the variation of mass transfer coefficientwith particle size for various levels of the intrinsic rate parameter;

FIG. 4 is a schematic showing the eight step VPSA cycle used in theexamples of the invention;

FIG. 5 is a graph showing VPSA performance for 15 s and 60 s cycles as afunction of nitrogen mass transfer coefficient using LiX adsorbent;

FIG. 6 is a graph showing the effect of particle size and cycle timeupon VPSA performance at a fixed intrinsic rate; and

FIG. 7 is a graph showing preferred cycle time/particle sizecombinations for various N₂ intrinsic rates.

DETAILED DESCRIPTION OF THE INVENTION

The invention is based upon the recognition that sorption rates ofadsorbent materials have a significant impact upon process performance,and that greater sorption rates can be affected by the propercombination of the intrinsic rate property (ε_(p)D_(p)) and the particlesize.

The objects of the invention are accomplished by implementing higherrates of mass transfer and by combining these with fast cycles andshallow beds. The preferred adsorption rate is established through acombination of the internal physical or intrinsic mass transfer rateproperties of the adsorbent particle and the particle size in such a wayto achieve significantly improved overall process performance. By theterm “sorption rate” we mean the rate at which the adsorbate loadingchanges in a given time period in an adsorbent particle for a givenadsorption separation process. This sorption rate is approximatelyproportional to the inverse of particle radius squared and is directlyproportional to the product of the “effective diffusivity” and theparticle porosity. An increase in this intrinsic rate property coupledwith a decrease in particle size results in an increase in adsorptionrate. By the term “intrinsic rate property” we mean the transportproperty that is due to the intrinsic characteristics of the adsorbentparticle including, but not limited to the structure, size, shape andlength, etc. of the macropores. Ideally, a material's intrinsic rateproperty is independent of particle size. In a preferred embodiment, theadsorption rate increases as the cycle time and bed depth decrease.

In practicing the invention, improved process efficiency is obtained byfirst affecting the largest sorption rate of the adsorbent that can bepractically attained through modification of that material's internalphysical properties, followed by an additional increase in adsorptionrate through reduction in the adsorbent particle size. The necessaryparticle size is related to the required overall mass transfer ratecoefficients and the cycle time/bed depth that lead to the lowestproduct cost. This strategy reduces the particle size only as much as isnecessary to achieve high performance. This leads to the use of thelargest particle size that satisfies the rate criteria, therebyresulting in the smallest bed pressure drop for given intrinsicproperties of the adsorbent.

By increasing the mass transfer rate according to the invention, one mayreduce the size of the mass transfer zone (L_(MTZ)) relative to the beddepth (L) and consequently, increase the working capacity of theadsorbent bed. Ackley et al. (WO 99/43416) have addressed a similarproblem by specifically providing higher effective macroporediffusivity, i.e. D_(p)≧3.5×10⁻⁶ m²/s in conjunction with reducedparticle size. Such prior art fails, however, to consider thesynergistic advantages of the combined effects of the intrinsic rateproperty and the particle size upon the reduction in the size of themass transfer zone.

Other prior art simply teaches that smaller particles lead to shortertransfer zones, which in turn facilitate shorter beds and faster cycles.However, there are several problems that occur with decreasing particlesize. First, pressure drop per unit bed length (ΔP/L) increases, withdecreasing particle size. This results in an increase in overall bedpressure drop ΔP, unless the bed depth (L) is decreased. Further, onsetof fluidization occurs at decreasing flow velocities as the particlesize decreases. Although velocity can be reduced by increasing frontalbed area to lessen both the increase in pressure drop and the onset offluidization, there are limitations to such area increases and in allcases a reduction in feed velocity generally results in a decrease inbed utilization. Finally, smaller particles are more difficult toimmobilize and retain in the adsorber.

The present invention, on the other hand, recognizes that processperformance is linked directly to mass transfer rate. In particular,such performance is the result of the coupled effects of mass transferrate with process conditions such as cycle time, feed temperature andadsorption/desorption pressures. The invention further recognizes thatparticle size is only one of several adsorbent parameters effecting masstransfer rate and that the particle size required to achieve a desiredrate varies depending upon the intrinsic mass transfer rate propertiesof the adsorbent particle. Since the particle size alone does notestablish the rate characteristic of the adsorbent, specification ofthis parameter alone in the equilibrium and mass transfer zones does notinsure maximum process performance.

The present invention considers the coupling of the effects of masstransfer rates (and the associated particle properties), the cycle timeand the bed depth to significantly improve gas separation efficiency,i.e. improvements represented by increases in adsorbent productivity,decreases in process power requirements and/or increases in productrecovery. The methodology is especially applicable to the production ofoxygen in PSA processes incorporating N₂-selective adsorbents, e.g. typeX or type A zeolites or advanced adsorbents such as highly Li-exchangedtype X or other monovalent cation-exchanged zeolites. While thisinvention has been demonstrated for the case of air separation, thegeneral methodology applies equally well to other gas phase separations:(1) that depend upon differences in equilibrium adsorption selectivity;and (2) in which the mass transfer resistance is dominated by diffusionin the macropores of the adsorbent particle, i.e. pores of dimension atleast an order of magnitude greater than the diameter of moleculesdiffusing into or out of the particle. For zeolites, this dimension ison the order of 30 Å to 40 Å. For the purpose of this invention,macropores are defined as those pores in the range of approximately0.0030 m to 20 m which corresponds also to the range of measurement bythe standard mercury porosimetry method. Adsorbents may be deployed bythis invention in one or more distinct adsorption zones, e.g.pretreatment and main adsorbent zones. One or more adsorbents may becontained in each zone, and the zones do not have to be contained in thesame adsorbent vessel. The pretreatment zone is located nearest the feedinlet and its purpose is to remove any undesirable contaminants from thefeed stream. Typical contaminants in air separation include water andcarbon dioxide. Those skilled in the art will appreciate that zeolites,activated alumina, silica gel as well as other appropriate adsorbentsmay be utilized in the pretreatment zone. The main adsorbent zone ispositioned downstream of the pretreatment zone (during the adsorptionstep) and contains adsorbent(s) selective for the primary heavycomponent(s) in the feed. The pretreatment zone may be excluded if thereare no contaminants in the feed stream.

The processes of the invention are for the separation of at least twocomponents of a gas phase mixture. Such separations are affected bydifferences in the equilibrium adsorption capacities of the componentsin the main adsorbent, i.e. at least one component in the mixture ismore selectively adsorbed at equilibrium in comparison to the adsorptionof the other components. The present invention is not concerned withkinetic adsorption processes where the primary separation mechanismresults from differences in the diffusion rates of the components intothe adsorbent.

The prior art most often projects an idealized concept of the masstransfer zone in which L_(MTZ) is independent of the cycle time and beddepth. Due to the presence of gradients in temperature, pressure andadsorbate loading in the adsorbent bed throughout all steps of an actualbulk separation process however, the motion of the adsorption/desorptionmass transfer fronts is not ideal.

Since process performance declines as the ratio of L/L_(MTZ) decreases,the goal of performance improvement must be to maintain or increase thisratio. For air separation with highly-exchanged LiX zeolites, ratios ofapproximately 4.0 are desirable. Wankat (1987), however, teaches thatincreasing the ratio L/L_(MTZ) beyond a value of 2.0 to 3.0 results inminimal improvement in process performance. For the purpose of thediscussions below, the L_(MTZ) is evaluated at the end of the adsorptionstep.

L_(MTZ) is a consequence of the resistance to mass transfer whichdetermines the adsorption rate. A linear driving force (LDF) model (E.Glueckauf, Trans. Faraday Soc. 51, 1540, 1955) can be used to representadsorption rate$\left( \frac{\partial\overset{\_}{w_{i}}}{\partial t} \right)\text{:}$

$\begin{matrix}{{\rho_{b}\quad \frac{\partial\overset{\_}{w_{i}}}{\partial t}} = {k_{i}\left( {c_{i} - \overset{\_}{c_{s_{i}}}} \right)}} & \text{(A-1)}\end{matrix}$

where (w_(i)) is the average loading of adsorbate (i), ρ_(b) is thepacked density of the adsorbent in the bed, c_(i) and c_(si) are averageadsorbate gas phase concentrations in the bulk fluid and inside theparticle in equilibrium with the adsorbate loading, respectively. Theterm in brackets is the “concentration driving force.” k_(i) is the masstransfer coefficient which can be represented as a combination of theintrinsic diffusion properties of the adsorbent and adsorbent particleproperties as follows: $\begin{matrix}{k_{i} = \frac{15{ɛ_{p}\left( {1 - ɛ_{b}} \right)}D_{pi}}{r_{p}^{2}}} & \text{(A-2)}\end{matrix}$

where D_(pi) is the effective pore diffusivity, ε_(p) is the voidfraction inside the particle, ε_(b) is the interparticle bed voidfraction (void fraction external to the particles) which for thepurposes of this disclosure varies from 0.34 to 0.37, and r_(p) is theparticle radius. The geometry of the macropores is imbedded in the termD_(pi), i.e. a tortuosity factor (τ) is commonly included in thedenominator of Equation A-2, but this term has been imbedded in D_(pi)for this disclosure. Equations A-1 and A-2 provide a convenient model torelate the various parameters that effect adsorption rate. While othermechanisms such as micropore diffusion, surface diffusion, filmdiffusion and bulk phase axial diffusion may influence the mass transfercoefficient, macropore diffusion dominates for many importantseparations, including air separation using type-X zeolites.

The component mass transfer coefficients k_(i) can be determined byfitting the appropriate model to data obtained from a breakthroughexperiment. Since ε_(p), ε_(b) and r_(p) can all be determined bymeasurement, the effective diffusivity Dpi is extracted from EquationA-2. This methodology and Equation A-2 clearly distinguish the effectsof intrinsic properties from the particle size upon adsorption rate. Forthe present invention, the product (ε_(p)D_(p)) represents the intrinsicrate property of the adsorbent. When combined according to Equation(A-2) with the appropriate particle size, a rate coefficient (k_(N2)≧12s⁻¹) can be obtained.

Equations A-1 and A-2 represent only one of several ways to characterizeadsorption rate. The precise description or definition of adsorptionrate is not critical as long as the description is consistently appliedand reflects the dominant mechanisms involved for the separations ofinterest. Such separations include those dominated by equilibriumeffects and mass transfer dominated by macropore diffusion in theparticle. The effective diffusivity (D_(pi)) is anempirically-determined parameter, and determination must be consistentwith the characterization of the adsorption rate.

Since the sorption rate associated with a given adsorbent's internalphysical properties has been quantified for the examples herein, thebreakthrough experiment employed to obtain the mass transfer ratecoefficients and effective diffusivities is briefly described below. Oneskilled in the art will recognize that variations of this experiment maybe used as long as the guiding criteria stated above are followed.

For the process of air separation, a breakthrough test is performed intwo steps in which the flow rate, pressure and temperature of the feedgas are the same in both steps. With reference to FIG. 1, this processwill be described. The first step involves saturation of the adsorbentbed 1 with O₂, the least selective component, provided via flow meter 2and line 3. In the second step, air or a synthetic air mixturecontaining N₂ and O₂ is then introduced to bed 1 via flow meter 4 andline 3. Valve 6 operates in conjunction with flow meter 2 such thatpressure of the air or synthetic air is maintained in an external loopuntil the four port valve 7 connects the air/synthetic air feed to line3 such that the air/synthetic air flows into bed 1. The pressure,temperature and composition of the feed mixture in the second stepshould be representative of that in an adsorption step of an actualprocess, e.g. 1.5 bar, 300° K and feed air composition. The molar fluxwas approximately 10 mol/m²s, although this flux may be varied asrequired. The pressure is maintained substantially constant in the bed 1by using a control valve 8 located on the downstream side of theadsorbent bed. The endspace and connecting piping volumes (dead volumes)are designed to be about 5% or less than that of the adsorbent bedvolume (approximately 20 cm³).

The flow rate and concentration of O₂ are continuously and accuratelymonitored throughout step two via flow meter 9 and oxygen analyzer 10until the breakthrough of N₂ is complete. Flow to analyzer 10 ismaintained at a fixed amount via fixed valve 5. In step two, the moreselectively adsorbed N₂ displaces the adsorbed O₂ already in the bed 1.As the bed nears saturation with the air mixture, the breakthrough of N₂is reflected in a decrease in the O₂ concentration and an increase inoverall flow rate of the effluent from the bed. The piping and adsorbentbed are maintained at the same temperature as the feed by immersing themin a thermostat bath controlled at the same temperature as the feed.

A computer model representing the test is then applied to simulate thebreakthrough test. A detailed adsorption model, based upon the governingmaterial and energy balances involved in the process, is incorporated.This model uses the same rate expression as that in Equation A-1. Themodel used here is represented by one dimensional plug flow withnegligible axial dispersion. Additional characteristics of the modelinclude: pressure drop (as determined by Darcy's Law or by the ErgunEquation), multicomponent isotherm (as determined by the loading ratiocorrelation), and adsorption rate (as determined by the linear drivingforce). A nonisothermal energy balance was used to account for heattransfer through the wall of the adsorbent bed. Simulations wereperformed by varying the mass transfer rate coefficients, k_(N) ₂ andk_(O) ₂ , until the breakthrough effluent flow rate and concentrationprofiles of the simulation matched those of the experiment.

The prior art has virtually ignored sorption rate effects due to amaterial's internal physical properties, taking such properties as fixedand invariant. The few exceptions are Moreau et al. (who did not addressdiffusivity nor the significant offsetting effects of high porosity), Luet al (who uses forced convection) and Ackley et al. (in which theprimary focus is upon increased effective diffusivity), each of whichhas been noted in the background above. Moreau et al. considered onlythe effect of porosity upon process performance, demonstrating that sucheffects were relatively small for 5A adsorbent in the conventional rangeof porosity (0.30≦ε_(p)≦0.38). As a result, Moreau teaches preferredporosity of ε_(p)>0.38. In Lu et al, the conditions favoringintraparticle forced convection are not particularly attractive to thepresent invention in which the intent is to increase the intrinsic rateproperty within conventional porosity levels while maintaining low bedpressure drop. The prior art of Ackley et al. (WO 99/43416) teachesimproved sorption rate through increased effective diffusivity(D_(p)≧3.5×10⁻⁶ m²/s). The mass transfer coefficient is adjusted throughthe combined effects of the effective diffusivity and the particle size.

The present invention arises in part out of the recognition that thesorption rate, more particularly the intrinsic rate property, can bemanipulated through specific formulation and/or processing of theadsorbent.

Examples of such formulations and/or processes include, the variation ofbinder content and type, the inclusion and subsequent burn-out of fibershaving selected dimensions, concentrations and compositions, causticdigestion of the binder and controlled drying and activation. Adsorbentshave been produced incorporating such methodologies and the resultingintrinsic rate property is greater than that for conventional,untreated/unmodified adsorbents. Further, such increases have beenachieved while maintaining the porosity within the desired range ofconventional adsorbents ε_(p)<0.38). Examples of such formulations aredisclosed in Chao et al. (WO 99/43415).

Unexpectedly high gains in process performance were realized when theintrinsic rate parameter was increased. This is evident in FIG. 2 whereboth normalized product recovery and bed size factor (BSF) are shown asfunctions of the intrinsic rate property (ε_(p)D_(p)) for N₂ and masstransfer rate coefficient (k_(N) ₂ ). Surprisingly for anequilibrium-dominated separation, there is nearly a 20% improvement inperformance when the intrinsic rate parameter (determined using theapparatus of FIG. 1, and at 300° K and 1.5 bar) increases from 7.7×10⁻⁷m²/s to 3.6×10⁻⁶ m²/s. Further, improvement in process performancediminishes as the rate coefficient is increased beyond some level. Asshown in FIG. 2, for a 60 second cycle (defined below), improvements inBSF and O₂ recovery lessen beyond the mass transfer coefficient k_(N) ₂=40 s⁻¹. Therefore it is preferred, for a 60 second cycle, to maintainthe mass transfer coefficient range between

12 s⁻¹≦k_(N) ₂ ≦40 s⁻¹. These results were obtained by simulation of aVPSA air separation process for the production of O₂ using highlyexchanged LiX (2.0) adsorbent having greater than 95% Li. The particlesize was held constant along with process conditions while the intrinsicrate parameter was varied for the results shown in FIG. 2. We shouldnote that, as will be discussed below, for shorter cycles this range ispreferred to be k_(N) ₂ ≧40 s⁻¹.

The desired rate coefficient can be obtained by manipulating theintrinsic properties of the adsorbent and/or the adsorbent particlesize. The effect of particle size upon the N₂ mass transfer coefficientfor various values of the intrinsic rate parameter (ε_(p)D_(p)) isillustrated in FIG. 3. Equation A-2 was applied to construct the familyof (ε_(p)D_(p)) characteristics shown.

It is apparent from the results in FIG. 3 that the larger the intrinsicrate parameter, the larger the particle size that can be used to achievea desired mass transfer coefficient. For example, a desired masstransfer rate coefficient k_(N) ₂ =40 s⁻¹ can be obtained with a 0.85 mmparticle size when the intrinsic rate parameter for N₂ is 7.7×10⁻⁷ m²/sor with a 1.95 mm particle size when the intrinsic rate parameter for N₂is 3.6×10⁻⁶ m²/s. We should note that when we refer to particle size(d_(p)), we refer to the average diameter of an adsorbent comprisingparticles distributed over a range of sizes, as those skilled in the artwill recognize.

The synergistic effects of coupling the intrinsic rate parameter withchanges in the particle diameter are evident from the resulting increasein the mass transfer rate coefficient. This effect is demonstrated bycomparing conventional and improved-processing LiX (2.0) adsorbent.

LiX (2.0) containing clay binder, produced in beaded form byconventional methods and without caustic digestion, was evaluated as areference sample (R-1). This adsorbent had an average particle diameterof 1.6 mm and an average porosity of 0.33. Breakthrough tests wereconducted and simulations were performed for an air feed to determinethe mass transfer rate coefficients according to the methods describedabove. Effective diffusivities were extracted using Equation (A-2). Theeffective N₂ pore diffusivity was determined by these methods to be2.7×10⁻⁶ m²/s, while the intrinsic rate parameter ε_(p)D_(p)=0.89×10⁻⁶m²/s. The resulting N₂ mass transfer coefficient was 11 s⁻¹.

To demonstrate the features of the present invention, a LiX (2.0)adsorbent (sample T-1) was produced using caustic digestion to convertbinder to zeolite. Care was taken in the activation of this material tominimize hydrothermal damage to the zeolite. Such caustic digestion andactivation treatments are well known to those skilled in the art ofzeolite manufacture. The size distribution of this adsorbent can beclassified as 10×20 (US Sieve Series), while the average particlediameter of the distribution was 1.3 mm. The average porosity (D_(p)) ofthis material was 0.35. The N₂ mass transfer coefficient and theeffective N₂ pore diffusivity were determined (by the methods referencedabove) to be 24 s⁻¹ and 3.15×10⁻⁶ m²/s, respectively. The parameterε_(p)D_(p)=1.1×10⁻⁶ m²/s.

The effect of the decrease in the particle size from 1.6 mm to 1.3 mm isexpected to increase k_(N2) from 11 s⁻¹ to approximately 17 s⁻¹, usingEquation (A-2). The remainder of the increase in mass transfercoefficient to k_(N2)=24 s⁻¹ for adsorbent T-1 is the result of themodest increase in the intrinsic rate parameter ε_(p)D_(p). Without theparticle size change, the increase in ε_(p)D_(p) would have increasedk_(N2) by only about 24%, e.g. from 11 s⁻¹ to 13.6 s⁻¹. When combinedwith a modest decrease in particle size, the intrinsic rate parameterprovides an amplifying effect and the rate coefficient is more thandoubled. Thus, the combination of modest decrease in particle size andmodest increase in intrinsic rate parameter results in a synergisticeffect that provides a substantial improvement in mass transfer ratecoefficient.

The sample T-1 was separated into individual size fractions and eachfraction was evaluated according to the same methods used to examine thedistributed T-1 adsorbent. The intrinsic properties ((□_(p) and D_(p))of each size fraction remained essentially the same as the average ofthese parameters for the T-1 material. The results for k_(N2) as afunction of particle diameter are shown in FIG. 3. The proportionalitybetween k_(N2) and the inverse square of r_(p) in Equation A-2 issupported by these data, noting that k_(N2) is determined by fitting thesimulated breakthrough response to the experimental breakthrough data.FIG. 3 shows that preferred values of the N₂ mass transfer coefficient(k_(N2)≧12 s⁻¹) can be achieved for this adsorbent (T-1) for averageparticle sizes d_(p)≦1.9 mm. A more-preferred N₂ mass transfercoefficient (k_(N2)≧20 s⁻¹) can be achieved for average particle sizesd_(p)≦1.5 mm. Mass transfer coefficients as high as 40 s⁻¹ for N₂ can beobtained at reasonable average particle sizes d_(p)≦1.0 mm. Byreasonable particle size, it is meant that both the retention ofadsorbent in the bed and the bed pressure drop can be maintained atreasonable levels using conventional methods and without costly and/orextreme measures.

To demonstrate the effects of mass transfer rate upon processparameters, VPSA air separation process performance was determined fortotal cycle times of 60 s and 15 s. The following conditions weremaintained: O₂ product purity at 90%, adsorption pressure at 1.5 bar,desorption pressure at 0.3 bar and feed temperature at 320° K. A simpleeight-step cycle including pressurization, feed, purge, equalization andevacuation was employed. A highly exchanged (>95% Li) LiX adsorbent(SiO₂/Al₂O₃ ratio of 2.0) was used as the main adsorbent with bed depthsof 1.37 m and 0.343 m for the 60 s and 15 s cycles, respectively. Themass transfer coefficients (determined by methods described previously)for O₂ were approximately 35% of those for N₂ for LiX adsorbents. Theaverage feed air molar flux was 17 mol/m²s.

The cycle is described in the diagram in FIG. 4, while the step timesare given in Table I for the 60 s cycle. The step times were allshortened by the ratio 15/60 for the 15 s cycle.

TABLE I 8-Step VPSA Cycle Step No. Description Duration(s) 1.00Feed/Make Product 6.00 2.00 Feed/Make Product/Purge 7.00 3.00Equalization Down 4.00 4.00 Blowdown/Evac. 13.00 5.00 Evacuation 6.006.00 Purge/Evac. 7.00 7.00 Equalization Up 4.00 8.00 Repressurization13.00

A detailed computer model of the process was used to determine theperformance at various levels of adsorption rate. The adsorbent bedmodel equations are similar to those described above for the model ofthe rate test. The energy balance for the adsorbent is adiabatic,however, in the process model. The different bed pressure drops in eachstep of the cycle were maintained nearly constant for all processexamples.

The simulation results for O₂ recovery and BSF are shown in FIG. 5a andFIG. 5b. As illustrated therein, product recovery drops considerably forN₂ mass transfer coefficients k_(N) ₂ <20 s⁻¹ for the 60 s cycle, whilea similar decline in performance is apparent for k_(N) ₂ <40 s in the 15s cycle. The BSF results reflect similar conclusions. Likewise,performance gains diminish significantly for k_(N) ₂ >40 s⁻¹ and k_(N)₂ >80 s⁻¹ for the 60 s and 15 s cycles, respectively. Increasing theadsorption rate alone nearly doubles the product recovery and halves theBSF, while the shorter cycle alone results in a BSF reduction of morethan a factor of three with only a minor penalty in O₂ recovery. Whencombined, the effects of higher adsorption rate and short cycle timelead to a reduction in BSF of more than a factor of six. The values ofthe N₂ rate coefficient (k_(N2)) leading to high performance in FIG. 5reflect the process conditions stated: O₂ product purity at 90%,adsorption pressure at 1.5 bar, desorption pressure at 0.3 bar and feedtemperature at 320° K for a feed composition of air. One skilled in theart will recognize that the same methodology described above can beapplied to determine the preferred rate constants for other processconditions.

In general (for cycle times less than about one minute) a mass transfercoefficient k_(N2)≧12 s⁻¹ is preferred, with a rate constant ofk_(N2)≧20 s⁻¹ being more preferred. As preferred values of k_(N2) are,in part, dependent upon cycle time (as shown in FIG. 5), for shortercycle times, greater values of k_(N2) are preferred. Thus for a cycletime of 15 sec, a rate of k_(N2)≧40 s⁻¹ is also preferred, and a valueof k_(N2) up to 80 s⁻¹ is acceptable.

FIG. 5, which is consistent with the results of FIG. 2 and the preferredranges set forth above, shows the best ranges of the adsorption ratecoefficient for cycle duration from 15 s to 60 s. As indicated inEquation A-2, such rates may be achieved by increasing the effectivediffusivity (DP) or the intraparticle void fraction (ε_(p)) and/or bydecreasing the particle size. Although each approach has theoreticallimits, decreasing particle size or increasing porosity are accompaniedby penalties to overall separation performance. Decreased particle sizeresults in increased pressure drop per unit bed length, increasedpotential for fluidization and greater difficulty in particle retentionin the bed as described above.

Intraparticle void fraction (ε_(p)) is defined by Equation A-3:

ε_(p)=ρ_(p)ν_(i)  A-3

where ρ_(p) is the particle density, and ν_(i) is the internal macroporevolume per unit mass of particle. ν_(i) may be determined by thewell-known mercury porosimetry method.

Increasing the porosity or intraparticle void fraction reduces theoverall active adsorbent content of the particle resulting in lowerparticle density. This in turn increases the volume of adsorbentrequired for a given N₂ adsorbate capacity (mol/g). There is a naturaltendency for the particle density to decrease as pore volume isincreased for a fixed adsorbent composition. Conversely, the pore volumeusually decreases when ρ_(p) increases. This apparent inverserelationship between particle density and macropore volume, while not inconstant proportion, tends to restrict the practical range over whichthe void fraction ε_(p) can be varied for each particular type ofadsorbent. Indeed, the intraparticle void fraction (ε_(p)) of commonsynthetic zeolites is typically in the rather narrow range of 0.30 to0.38 (Wankat, P. C., Rate-Controlled Separations, Elsevier AppliedScience, 1990, pg. 226).

This range of porosities for zeolites is also related to a physicalstrength requirement, i.e. adsorbent particles in the bottom of largecommercial beds must resist crushing under the weight of thousands ofpounds of adsorbent contained in the adsorber vessel. High porosity/lowdensity particles are subject to lower crush strength. Larger internalvoid fraction (ε_(p)) also increases the non-selective gas storagevolume in the adsorbent bed and thereby decreases the separationcapability, i.e. reduces overall product recovery. While at first glanceincreasing ε_(p) appears to be a good way to increase adsorption rate(as indicated by Equation A-2), the offsetting effects in processperformance and the potential mechanical difficulties arising fromadsorbent particle breakdown make increasing porosity a limited choicefor rate enhancement.

Ackley et al. (WO 99/43416) suggests that the preferred method forincreasing adsorption rate is to increase the effective diffusivities(D_(p)) in the macropore space of the particle. Increased D_(p) aloneresults in higher mass transfer coefficients with virtually nooffsetting effects in performance or problems in decreased particlestrength. The maximum effective diffusivity is limited, however, to theMaxwell (free space) diffusivity, e.g. for N₂/O₂ at 293° K and 1.0 atm,this limit is 2.2×10⁻⁵ m²/s (Hirschfelder, J. O. et al., MolecularTheory of Gases and Liquids, John Wiley & Sons, 1964, pg. 579). Thislimit is significantly above the effective diffusivities for N₂/O₂ inconventional zeolites. Significant increase in the rate coefficient wasshown when combining D_(p)≧3.5×10⁻⁶ m²/s with modest decrease inparticle size. Ackley et al. discounted any potential benefits thatmight be obtained from increasing the porosity.

Increasing porosity beyond the conventional limit has a detrimentaleffect upon both the particle strength and the adsorbent's volumetriccapacity. Increasing the porosity within the conventional range((ε_(p)<0.38) results in only small to modest increases in the ratecoefficient. While decreasing particle size can substantially increasethe rate coefficient, such an approach is accompanied by undesirableincrease in adsorbent bed pressure drop. The present invention alsorecognizes that it may not be possible to control D_(p) and ε_(p)completely independent of each other. Indeed, a situation is envisionedwhere the predominant macropore space is contained in ink bottle-shapedpores such that only a weak relationship exists between the porediffusivity and the porosity. Conversely, D_(p) and ε_(p) could beclosely related when the macropores are of nearly uniform shape andsize, i.e. as shape and size change, both pore diffusivity and porositychange. This invention shows that the most effective method to increasethe rate of adsorption is to increase the intrinsic rate parameter(ε_(p)D_(p)) as much as possible, while maintaining ε_(p) within theconventional range, coupled with a controlled decrease in particle sizeto minimize increase in pressure drop. By exercising such control in thesimultaneous manipulation of ε_(p)D_(p) and particle size, a synergisticeffect results which allows an increase in the adsorption rate withoutsignificant penalties in particle strength, volumetric capacity or bedpressure drop. Furthermore, the increase in the rate coefficientachieved by the simultaneous manipulation of ε_(p)D_(p) and r_(p) ofthis invention is greater than can be achieved from the change in anyone or two of these parameters alone.

By way of illustration, several examples are provided in which theintrinsic rate parameter ε_(p)D_(p) has been increased throughformulation and/or processing of the aggregated product. The details ofthe formulation/processing are described in WO 99/43415 (Chao et al).The results demonstrate that intrinsic rate can be significantlyincreased in comparison to conventional adsorbents. Such improvement tothe intrinsic adsorbent properties can then be applied to greatadvantage in separation processes as described herein. These examplesare in no way limiting, but illustrative only, as one skilled in the artwill appreciate that alternative methods for achieving increasedintrinsic rate will lead to corresponding improvements in processperformance.

Chao (WO 99/43415) has demonstrated various formulations and methods forproducing adsorbents with intrinsic rate higher than that ofconventional adsorbents. The intrinsic rate of adsorbents can beenhanced by first combining a low amount of binder with zeolite in thebead-forming step followed by caustic digestion (c.d.). The intrinsicrate characteristics of the adsorbent can be improved further by theaddition of fiber with subsequent burnout. Not wanting to be restrictedto any one method or formulation, the detailed procedure for producingadsorbent S-1 of the invention is herein described as one example ofmaking such high rate adsorbents. The method of making S-1 involves thefour primary steps of bead forming, caustic digestion, ion exchange andcalcination as described below.

Bead Forming

2640 gm dry weight of NaKX(2.0) (wet weight 4190. gm) zeolite, 360 gmdry weight of the ECCA Tex-611 (wet weight 426 gm) kaolin clay weremulled for 15 min. while water was pumped in at a rate of 10 ml/min. Therate of water addition was then decreased to 4 ml/min for 40 min and themixture was mulled another 20 min. The mulled mixture was thentransferred to a DBY-10R Nauta Mixer (supplied by Hosokawa Micron PowderSystems) and mixed for about one hour. The lumps were broken down toreturn the mixture to a powder state. Water then was added slowly by anatomizer. As the moisture of the mixture increases, beads start to form.The growing of the beads was stopped by adding dried bonding mix at atime for harvesting the highest yield of 8×12 size beads.

The beads were dried in air overnight and then calcined in a Blue M ovenwith a dry air purge. The oven temperature was ramped up to 600° C. in 2hours and then held at 600° C. for 2 hours during the dry air purge.

Caustic Digestion

1861.8 gm dry weight of calcined NaKX(2.0) beads of size 6×16 with 12%binder were used for caustic digestion. To prepare digestion solution,360 gm of NaOH (9 mole) and 251.1 gm (4.475 mole) KOH was dissolved in7386 gm of water. To this solution, 320 ml of sacrificial NaKX2.0 beadswere added and stirred at 90 C. for 2 hours. The solution was left tosettle and 6397.7 gm supernatant was collected. To this supernatant,1477.2 ml of water, 72.0 gm of NaOH and 50.2 gm of KOH were added tomake up for the discarded caustic. The resulting solution was used asdigestion solution.

The beads were loaded into two stainless steel columns of 3 inchdiameter and the solution from a common reservoir was recycled througheach column at a flow rate of 30 ml/min. and temperature of 88° C. for26 hours. After digestion the beads were washed by pumping 40 liter ofNaOH solution (pH=12, 88 C.) through each column. The beads in eachcolumn were further washed with 30 liter of NaOH solution (pH

=8.5, 88° C.). The product, NaKX2.0CD, was air-dried and screened tovarious particle size fractions.

Ion Exchange

694.5 gm dry weight of NaKX(2.0)CD 8×12 beads were loaded into a 3 inchi.d. glass column. A 10 inch layer of 3 mm Pyrex glass beads was placedat the bottom of the column to serve as a preheating zone for thesolution. The column was wrapped with a heating tape. The ion exchangesolution was first passed through a 15 liter 90° C. preheating flask topartially remove any dissolved air to prevent air bubbles from formingthat could be subsequently trapped in the column. The hot solution wasthen pumped into the bottom of the column.

The ion exchange solution was prepared by dissolving 2162 gm LiCl in 80liter distilled water (0.64M) then LiOH solution was added to adjust pHof solution to 9. The solution was pumped through the column at thespeed of 15 ml/min. until ten to twelve times the stoichiometric amountof LiCl, for full Li-exchange of the beads, had been circulated throughthe column. After the ion exchange was completed, the product was washedwith 30 liter of 90° C. distilled water at a flow rate of 60 ml/min. ThepH of this water was adjusted to 9 by adding LiOH.

Drying and Calcination

The washed product was first air-dried and then dried further in a lowtemperature oven with ample air purge for 3 hours to bring the moisturecontent of the beads to about 12-15%. The dried beads were calcined in aBlue M oven with ample dry air purge. The oven temperature was rampedfrom room temperature to 600° C. in two hours and maintained at 600° C.for 40 minutes. The sample was removed from the oven at 450° C. andplaced into a tightly sealed glass jar for cooling.

The N₂ mass transfer coefficients and intrinsic rate parameters(ε_(p)D_(p)) were determined from breakthrough tests as described abovefor commercial zeolites available in bead form as 13XHP, 5AMG, LiX(2.5)and LiX(2.3) from UOP of Des Plaines, Ill. USA. These results aresummarized in Table II for reference adsorption conditions of 1.5 barand 300° K. The relatively narrow range of the intrinsic rate parameterfor these commercial materials is reflective of the conventionalprocessing methods for zeolites. The greater range of mass transfercoefficients for these same adsorbents occurs almost entirely from thedifferences in particle size.

Several different treatments were applied to LiX (2.0) zeolite in orderto enhance the effective diffusivities for N₂ and O₂. As describedabove, various clay binder types and contents, and conversion of thebinder to zeolite through caustic digestion (c.d.) and the use of afiber additive with subsequent burnout were all explored. The effects ofthese treatments are illustrated in Table II for LiX (2.0) adsorbents(S1-S4).

TABLE II Summary of Adsorbent Properties Bind- Zeolite er (SiO₂/ Con-ε_(p)D_(p) d_(p) k_(N2) Sample Al₂O₃) tent Other ε_(p) m²/s mm s⁻¹ 13XNaX (2.5) 0.31 9.92 × 10⁻⁷ 2.1 8 5A MG NaCaA 0.32 9.92 × 10⁻⁷ 0.7 80Oxysiv-7 LiX (2.5) 0.36 9.36 × 10⁻⁷ 0.55 100 S-0 LiX (2.3) 0.33 9.57 ×10⁻⁷ 1.9 10 S-1 LiX (2.0) 12% c.d. 0.35 1.93 × 10⁻⁶ 2.0 18 S-2 LiX (2.0)20% c.d. 0.27  5.4 × 10⁻⁷ 2.0 5 S-3 LiX (2.0) 20% w/fi- 0.32 1.09 × 10⁻⁶2.0 10 ber c.d. S-4 LiX (2.0) 12% 0.35 2.77 × 10⁻⁷ 2.0 2.6

Sample S2 represents a zeolite formulation using 20% clay bindersubsequently treated by caustic digestion (c.d.) to convert binder tozeolite. The resulting porosity is about 10% lower and the intrinsicrate parameter is about 46% lower for S2 than these same properties for13X and 5A MG. When the binder content is lowered to 12%, there is asubstantial difference in the intrinsic rate as a result of the causticdigestion and conversion of binder to zeolite. The c.d. sample S1 has anintrinsic rate nearly seven times greater than that for sample S4 (noc.d.) with no significant difference in the porosities. Furthermore, theintrinsic rate parameter of sample S1 is two times greater than that ofLiX 2.3 adsorbent (S0)—also with only a small change in porosity. Interms of rate coefficient, k_(N2) of S1 is 1.8 to 7.0 times larger thanthat of any of the other materials of equal particle diameter in TableII. Samples S1 -S4 all use the same type of clay binder (kaolin). Insamples S1 -S3, this binder is converted to zeolite by causticdigestion. Although samples S1 -S3 have essentially the same finalchemical composition (LiX (2.0)), the intrinsic rate parameters forthese samples vary widely due to the different pore structures createdas a result of the different formulation and processing steps.

It is evident from these results that substantial increases in intrinsicrate can be obtained through special formulation and processing of theadsorbent. Additionally, these improvements in intrinsic rate areobtained while maintaining the adsorbent particle porosity in the samerange as conventional adsorbents. Such increases in this intrinsic rateproperty of the adsorbent can then be coupled with the proper choice ofparticle size and process operating conditions to achieve significantprocess performance advantages—subsequently captured as a reduction inthe overall cost of the product.

Once the sorption rate associated with a material's internal physicalproperties is obtained within the manufacturing and cost constraints ofa given methodology, the results of FIG. 5 are combined with thecharacteristics of FIG. 3 to select the particle size necessary toachieve the desired mass transfer rate coefficient; i.e. a ratecoefficient that leads to high process performance and minimum productcost. A value of ε_(p)D_(pN2)=1.81×10⁻⁶ m²/s for a LiX(2.0) (>95%exchanged) adsorbent is selected to illustrate the concept. Along thisε_(p)D_(pN2) characteristic in FIG. 3, particle diameters of 1.85 mm,1.3 mm and 0.92 mm correspond to values of k_(N2)=20 s⁻¹, 40 s⁻¹ and 80s⁻¹, respectively. Using a bed of 1.37 m depth containing 1.85 mmparticles as a reference condition for pressure drop, the bed depths forthe smaller particle size configurations are now established from theErgun equation to keep the overall bed pressure drop the same in allthree cases. Note that a lower pressure drop could have been chosen as areference condition. The cycle time was then adjusted to maintain aminimum product purity of 90% O₂. In the first three cases the endspacevolumes (void space above and below the adsorbent bed inside the vessel)were maintained constant. The results of process simulations are shownin Table III and FIG. 6.

TABLE III Summary of Simulation Results: Bed Depth/Cycle Time/ParticleSize Study (P_3) (R_2) (R_3) (R-3rv) O₂ Purity (%) 90. 90. 91. 90. O₂Recovery 0.55 0.52 0.45 0.54 Throughput TPDO 58.00 55.00 48.00 57.00 BSFlb/TPDO 737.00 387.00 224.00 187.00 k_(N2) s⁻¹ 20.00 40.00 80.00 80.00Particle diameter d_(p), mm 1.85 1.30 0.92 0.92 bed depth, m 1.37 0.690.34 0.34 Lower endspace void fraction 0.14 0.30 0.59 0.14 Upperendspace void fraction 0.18 0.38 0.76 0.18 Cycle time, sec. 60 30 15 15

The first three columns in Table III show the significant reduction (upto 70%) in BSF that is achieved with the shorter cycles enabled byhigher mass transfer rates. Unfortunately, the product recovery andthroughput decline substantially for the R_(—)3 case with 0.92 mmparticle diameter and a 0.343 m bed depth. This is very undesirable dueto the negative impact of reduced recovery upon power consumption.

A major contributor to this problem is the increasing fraction ofendspace void volumes relative to bed volume as the bed depth (and bedvolume) decrease as shown in Table III, i.e. the upper and lowerendspace void fractions increase from 0.18 and 0.14 to 0.76 and 0.59,respectively. A fourth case (R_(—)3 rv) was simulated with a reductionin endspace volumes to restore the fractional endspace voids to the sameas that in the 1.37 m bed depth reference case (P_(—)3). The productrecovery and throughput are nearly fully restored. Thus, it is preferredto maintain the void fraction of each of the endspaces at 30% or lessthan the total adsorbent bed volume. As the intrinsic rate parameterincreases, the performance characteristics in FIG. 6 shift to the right,i.e. similar short-cycle performance gains are realized at even largerparticle sizes.

The information in FIGS. 3, 5 and 6 are now combined with the conceptsof the invention and the example results to define the preferredparticle size and intrinsic rate, i.e. combinations that will result inthe highest bed utilization and overall best process performance forvarious cycle times for VPSA air separation using LiX (2.0). The resultsare shown in FIG. 7 for the range of intrinsic rate parameter for N₂,7.66×10⁻⁷≦ε_(p)D_(pN2)≦4.0×10⁻⁶ m²/s.

From FIG. 7, it is evident that the larger the intrinsic rate parameter,and thereby the larger the sorption rate derived from this internalphysical property of the adsorbent, the larger the particle size thatcan be accommodated to achieve a desired performance, e.g. for a cycletime of 30 s, a particle size within the range of about 0.85 mm to about2.0 mm, preferably between about 1.1 mm and about 1.6 mm will berequired depending upon the intrinsic rate properties of the adsorbent.Thus, when the intrinsic rate parameter ε_(p)D_(pN2)=1.81×10⁻⁶ m²/s, aparticle diameter of 1.3 mm is recommended for a cycle time of 30 swhile a particle diameter of 1.75 mm is best for 55 s cycle.

Considering the results of FIGS. 5-7, it is preferred that the ratecoefficient (k_(N2)) be coupled with a ε_(p)D_(pN2) of greater than orequal to 1.1×10⁻⁶ m²/s, preferably 1.3×10⁻⁶ m²/s, more preferably1.5×10⁻⁶ m²/s.

Although FIG. 6 indicates that the highest bed utilizations (lowest BSF)correspond to the shortest cycles, there may be compelling design andcost reasons to operate above the shortest cycle times, e.g. the lowestproduct cost may not correspond to the shortest cycle if endspace voidvolume cannot be controlled in the desired range, valve cycle times maylimit the shortest cycle times, etc. For these reasons, FIG. 7 providesa guide for these parameters over a significant cycle time range. Ingeneral, the bed depth will scale directly with the cycle time asillustrated in Table III.

To accommodate a range of desirable particle sizes: for a cycle time ofless than or equal to 80 s, the bed depth is preferably less than orequal to about 2.0 m; for a cycle time of less than or equal to about 60s, the bed depth is preferably less than or equal to about 1.5 m;similarly for a cycle time of less than or equal to 40 seconds, the beddepth is preferably less than or equal to 1.2 m; and for a cycle time ofless than or equal to about 20 s, the bed depth is preferably less thanor equal to about 0.63 m.

As indicated above, an object of the present invention is to makesignificant improvements in adsorbent utilization and product recoverythrough enhancement of the rate characteristics of the adsorbent.—Thiscan be achieved primarily from a combination of an increase in theintrinsic rate parameter (ε_(p)D_(p)) and the proper selection of theaverage particle diameter. The improved recovery achieved under theconditions of the invention also leads to reduced power consumption perunit of product produced. The invention is preferably directed atequilibrium-based adsorption separation processes with mass transportdominated by intraparticle pore diffusion. While the examples have beendirected at air separation using a single main adsorbent, the inventionis not limited to binary mixtures, nor to air as a feed nor to a singlemain adsorbent.

Further, when more than a single separation is to be achieved, it iscontemplated to include one or more adsorbents as main adsorbents. Insuch a case, each adsorbent would be responsible for a differentseparation or a different level of the same separation. Multiple masstransfer zones may then be present in the process. An analysis similarto that described above would be performed for each of theadsorbent/adsorbate combinations where overcoming significant masstransfer resistance limitations would lead to overall improvements inprocess performance. Thus, the properties (particularly those related tothe rate of adsorption) of the different adsorbent materials in the mainadsorbent zone are selected to maximize all of the separations requiredof the process. Examples of such processes include the recovery of H₂from H₂/CO/CO₂/CH₄ mixtures; prepurification, including the removal ofH₂O and CO₂ from air; separation of Ar from air or N₂ or O₂; drying ofprocess streams; and the recovery of CO₂ from flue gases or from H₂ PSAtail gas.

Type X zeolite adsorbents are suggested for air separation, mostpreferably highly-exchanged LiX as described by Chao (U.S. Pat. No.4,859,217). Other type X materials with monovalent cations or mixedcations are also applicable to the present invention such as thosesuggested by Chao (U.S. Pat. No. 5,174,979). The invention is alsoapplicable to any type of equilibrium-selective adsorbent materialincluding, but not limited to, A-zeolite, Y-zeolite, chabazite,mordenite, clinoptilolite and various ion exchanged forms of these, aswell as silica-alumina, alumina, silica, titanium silicates and mixturesthereof.

It should also be clear that the present invention can be practiced withvarious deployments of adsorbents in the main adsorbent zone, e.g.layers and mixtures of adsorbents of various types or of the same typebut with varying adsorption and/or physical characteristics. Forexample, the enhanced rate concepts of this invention could be appliedto the layered beds suggested by Ackley in U.S. Pat. No. 6,152,991, aswell as Notaro et al (U.S. Pat. No. 5,674,311) and Watson et al (U.S.Pat. No. 5,529,610).

Finally, a further improvement over the basic invention can be obtainedby distributing the adsorbents with different rate properties tominimize pressure drop and/or mass transfer zone size. The selection ofproperties should be made in order to increase the rate of adsorptionand minimize the fractional size(s) of the mass transfer zone(s) at theend of the adsorption step.

The present invention teaches a method to improve process performance byreducing mass transfer limitations while minimizing any increase inprocess pressure drop. Bed depth and cycle time are reduced tocompensate for increased specific pressure drop (pressure drop per unitdepth of adsorbent) when particle size is reduced. There may be cases,however, where either a further reduction in pressure drop is desiredand/or where the use of adsorbents with different rate properties isdesirable or necessary. In such an embodiment, a poorer sorptionrate-quality adsorbent (low mass transfer coefficient) could be used inthe equilibrium zone and a higher sorption rate-quality version of thesame adsorbent (high mass transfer coefficient) in the mass transferzone.

It is further contemplated that the poorer rate-quality material in thislatter condition could also be of smaller diameter. This would result ina configuration with regard to particle sizes in the adsorbent bed thatis completely opposite to the prior art teachings. Thus when multipleadsorbents with different rate characteristics must be used, maintainingthe adsorbent with the largest mass transfer rate coefficient in themass transfer zone insures the best overall process performance.

Since the mass transfer zone forms initially and develops in whateventually becomes the equilibrium zone (at the end of the adsorptionstep), the rate of adsorption cannot be too low relative to that in asucceeding layer of adsorbent. This is because the leading edge of themass transfer zone would erupt from the adsorber before the trailingedge crosses the boundary between the two materials. This would resultin a reduced size of the equilibrium zone and increased size of the masstransfer zone and consequently, overall lower product recovery and/orpurity.

This condition may be minimized by selecting the adsorbents and the masstransfer coefficients (MTC) of the most selective component such thatthe size of the mass transfer zone in the adsorbent of the lowest MTC isno more than twice that of the size of the mass transfer zone in theadsorbent of the highest MTC.

The problem may also be solved by distributing the adsorbents in such away as to achieve a gradual increase in mass transfer coefficients (incontrast to discrete layers) from the inlet to the outlet of theadsorber. When multiple adsorption zones are contained in the mainadsorbent for the purpose of multiple separations, it is appreciatedthat the concept of mass transfer coefficient gradients (either bydiscrete layers or by gradual change) can be applied individually toeach included separation zone.

The concepts of this invention are not limited to any specific set ofprocess conditions but may be applied over a wide range of processconditions, e.g. temperatures, pressures, feed velocities, etc. It isonly necessary to evaluate the rate characteristics of the adsorbent atthe process conditions of interest before applying these concepts inorder to insure maximum process performance. Likewise, these conceptscan be applied to single-bed as well as multi-bed processes operatingwith subatmospheric (VSA), transatmospheric (VPSA) or superatmospheric(PSA) cycles.

In addition, the use of such materials would allow for operation ofPSA/VPSA/VSA processes at relatively low pressure ratios (e.g. the ratioof the highest adsorption pressure to the lowest desorption pressure),preferably less than 7.0 and more preferably less than 5.0, still morepreferably less than 4.0 and most preferably less than 3.0.

While the process examples disclosed in this application use aneight-step cycle, the benefits of the invention may also apply tosimpler cycles comprising fewer steps and more complex cycles comprisingadditional steps.

The enhanced-rate concepts described here are not limited to anyparticular adsorber configuration and can be effectively applied toaxial flow, radial flow, lateral flow, etc. adsorbers. The adsorbent maybe constrained or unconstrained within the adsorber vessel.

The benefits of the invention may also be obtained in cycles in whichthe primary product is the more selectively adsorbed component (e.g. N₂)or in cycles wherein both the more and less strongly held component arerecovered as product.

The term “comprising” is used herein as meaning “including but notlimited to”, that is, as specifying the presence of stated features,integers, steps or components as referred to in the claims, but notprecluding the presence or addition of one or more other features,integers, steps, components, or groups thereof.

Specific features of the invention are shown in one or more of thedrawings for convenience only, as such feature may be combined withother features in accordance with the invention. Alternative embodimentswill be recognized by those skilled in the art and are intended to beincluded within the scope of the claims.

What is claimed is:
 1. A process for separating a preferred gas from agas mixture containing said preferred gas and other less preferredgases, said process comprising passing said gas mixture over anadsorbent having a mass transfer coefficient (MTC) for nitrogen ofK_(N2)≧12 s⁻¹ and an intrinsic rate for N₂, when measured at 1.5 bar and300° K, of ε_(p)D_(pN2)≧1.1×10⁻⁶.
 2. The process of claim 1 wherein theadsorbent has particles having an average size dp of ≦1.9 mm.
 3. Theprocess of claim 1, wherein ε_(p)≦0.38.
 4. The process of claim 3,wherein ε_(p)≧0.30.
 5. The process of claim 1, whereinε_(p)D_(pN2)≦1.3×10⁻⁶.
 6. The process of claim 1, wherein said processcomprises adsorption and desorption steps, and wherein the pressureratio of the highest adsorption pressure to the lowest desorptionpressure is less than 7.0.
 7. The process of claim 6, wherein thepressure ratio is less than 5.0.
 8. The process of claim 6, wherein thepressure ratio is less than 4.0.
 9. The process of claim 6, wherein saiddesorption step takes place under subatmospheric conditions.
 10. Theprocess of claim 1, wherein the preferred gas is oxygen.
 11. The processof claim 1, wherein the gas mixture is air.
 12. The process of claim 1,wherein said process takes place in a radial flow, axial flow or lateralflow adsorber.
 13. The process of claim 12, wherein said adsorbent iseither constrained or unconstrained within said adsorber.
 14. Theprocess of claim 1, wherein said adsorbent is a selected from the groupconsisting of X-zeolite, A-zeolite, Y-zeolite, chabazite, mordenite,clinoptilolite, silica-alumina, silica, titanium silicates and mixturesthereof.
 15. The process of claim 1, wherein said adsorbent containslithium.
 16. The process of claim 1, wherein said adsorbent is anX-zeolite containing lithium.